The fastest way to boost this is simply set 3 round sessions and have the whole team kill themselves (preferrably by grenade).
Ideally this should be the last sought after achievement after a full cycle of nabbing every other online achievement, as boosting or going for those will nab you played sessions anyway.
If you keep at only 3 round sessions, this achievement will unlock at 600 listed heists joined on your stats page.
Make sure the session completes all the way to the closing scoreboard screen for the session, for each session to count. Your "heists joined" count will get severely messed up if you're involved with games where hosts quit.
Pro tip: Try not to keep track, and just let the achievement pop when it does, otherwise you'll drive yourself mad keeping count or constantly looking at the stats to check.
*EDIT 07-19-11* - If you have no intentions of going for the leaderboard achievements, you can do this on Ranked and keep track of your sessions by viewing the leaderboards. I wouldn't advise it though.